The following statement may be of interest to you. It was sent to me over a year ago when I was working on a 1/45 Gemini Titan with closer scale motors. The last paragraph should be of interest. You might would want to do some testing first if you want to consider two 18mm motors back to back. Would be a longer motor tube. Could use engine blocks when only using single motors. Just a idea, that’s all. I choose not to try it.

It was a rather short-lived "thing" in NAR competition in the 1976-79
> timeframe and was first demonstrated by Trip Barber. It allowed, for
> example, the use of a "B" class motor in a smaller diameter tube than
> "standard" 18mm B motors...obviously giving one an edge in competition. It
> was eventually prohibited from competition and quickly disappeared because
> it was both not a widely known modification and there was little need
> outside of competition. However, for serious scale modelers, competition or
> not, there is a desire in many cases to have the scale motors as true to
> scale as possible even with a model rocket motor housed inside.
>
> There were some interesting combinations of tandem motors (which, BTW, only
> works with "black powder" motors. For example, a standard 18mm motor fits
> inside a standard 24mm casing. This combination allowed use in most models
> designed for 24 mm motors without modification to accommodate the longer
> motor since the 18mm "front" motor would pass right through the engine block
> or forward engine hook tang. It also meant that you didn't have to have a
> slightly larger engine tube since there was no need to "overwrap" the joint
> with glass cloth. Of course, a smaller motor firing down through a larger
> motor reduced the sum of the impulse delivered by the two motors vs. if they
> were not "tandemed" but you still picked up performance nevertheless.
>
> Also, you didn't want to tandem a high thrust motor with a large nozzle
> above a lower thrust motor with a smaller nozzle. That would almost
> guarantee a rupture at the joint between the motors. Tests did show
> (surprisingly) that there was little performance lost by tandeming two
> identical engines. In other words, say, two 10n/sec C motors did in fact
> almost sum up to 20n/sec or basically a full D black powder motor in an 18mm
> casing (though it was twice as long).

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